The U.S. Federal Aviation Administration (FAA) warned on Monday airline passengers at New York City-area airports could see significant delays in the evening due to air traffic control staffing issues.

The FAA said "departure and arrival delays this evening could approach two hours" at John F. Kennedy (JFK), New York LaGuardia and Newark Liberty airports. The FAA also issued https://nasstatus.faa.gov a ground stop at LaGuardia set to last until 6:15 p.m.

FlightAware, a flight tracking website, said just over 600 flights at the three airports had been delayed on Monday, 16% of arriving and 11% of departing flights at Newark, about 18% of both arriving and departing flights at LaGuardia and around 15% of both at JFK.

A person briefed on the matter said the staffing issue was related to sick leave. The New York City area has the most congested airspace in the United States.

U.S. airlines have blamed a significant part of the summer travel disruptions, which have impacted tens of thousands of flights, on a lack of air traffic control staffing. In June, an airline trade group said FAA staffing issues were "crippling" East Coast traffic.

Rich Santa, who heads the National Air Traffic Controllers Association, said in July the FAA needs to do a better job of ensuring adequate staffing to oversee national airspace.

Santa said "unfortunately, FAA staffing is not keeping up with attrition."

Acting FAA Administrator Billy Nolen told Reuters in July the FAA was "on track to hire 1,000 controllers this year." For 1,500 open positions, the FAA accepted 57,956 applications for review.

Air traffic controllers work in control towers, approach control facilities, or route centers, and are essential for coordinating aircraft traffic between the nation's airports.

The FAA said last month "airlines' data show that the vast majority of delays are not due to air traffic controller staffing. Where demand has increased, the FAA is adding additional controllers."